The microbial desulfurization of fossil fuels has been an area of active investigation for over fifty years. The object of these investigations has been to develop biotechnology based methods for the pre-combustion removal of sulfur from fossil fuels, such as coal, crude oil and petroleum distillates. The driving forces for the development of desulfurization methods are the increasing levels of sulfur in fossil fuel and the increasingly stringent regulation of sulfur emissions (Monticello et al., "Practical Considerations in Biodesulfurization of Petroleum," IGT's 3d Intl. Symp. on Gas, Oil, Coal and Env. Biotech., (Dec. 3-5, 1990) New Orleans, La.).
Many biocatalysts and processes have been developed to desulfurize fossil fuels, including those described in U.S. Pat. Nos. 5,356,801, 5,358,870, 5,358,813, 5,468,626, 5,198,341, 5,132,219, 5,344,778, 5,104,801, 5,002,888, and Ser. No. 08/351,754, now abandoned (EBC94-08), incorporated herein by reference. Economic analyses indicate that one limitation in the commercialization of the technology is improving the reaction rates and specific activities of the biocatalysts, such as the bacteria and enzymes that are involved in the desulfurization reactions.
Current methods for the biodesulfurization of fossil fuels result in the formation of an aqueous sulfate solution. Disposal of the waste sulfate is a significant problem. For example, sulfate can be precipitated from solution by addition of an appropriate metal ion, but the resulting solid salt must be disposed of in a landfill. Alternatively, the sulfate can be transformed into another substance, such as hydrogen sulfide, which is then treated via conventional refinery processes. Both methods add significantly to the cost of biodesulfurized fuels.
There is, thus, a need for a method of desulfurizing fossil fuels using biocatalysis which does not produce sulfate or other by-products which present significant disposal problems.